1. Field of the Invention
This invention relates to signal receivers for converting a received modulated information signal to an IF signal and, more particularly, to such receivers that can be fabricated on an integrated circuit chip and used to receive either an FM signal, an AM signal, or both.
2. Description of Relevant Prior Art
It has been proposed to fabricate a combined FM/AM receiver on a single integrated circuit (IC) chip. One example is described in Japanese Patent Publication No. 62-48934, filed by the assignee of the instant invention. In that proposal, however, it is difficult, if not impossible, to fabricate the requisite intermediate frequency filter on the same IC chip as the remaining circuitry. This is because the standard IF frequency used in connection with FM broadcast signals is on the order of 10.7 MHz, whereas the standard IF frequency used in connection with AM broadcast signals is on the order of 455 kHz (or, in some instances, 450 kHz). It is appreciated that the wide range needed to accommodate both FM and AM IF frequencies is not easily achieved by a single band pass filter integrated with the FM/AM receiver. Hence, although a combined FM/AM receiver may be formed as a single IC chip, separate band pass filters tuned to 10.7 MHz and 455 kHz, respectively, are needed; and such band pass filters typically are constructed as a ceramic filter attached as an external circuit component to the FM/AM receiver chip.
The use of an externally attached IF filter is accompanied by various drawbacks. For example, this results in an increase in the number of peripheral parts or components which constitute the receiving device. Also, an external element, as opposed to a component simply included in an IC design, restricts the overall layout, or architecture, of the printed circuit board used with the device. Furthermore, the use of external circuits results in a receiver of larger size and thickness, and requires higher production and assembly costs. For example, a combined FM/AM receiver incorporated into a personal stereo audio device, such as a Walkman (registered trademark) audio device increases the size and production costs of that device.
Since the need for providing an externally connected IF filter arises out of the relatively high IF frequencies used, particularly when receiving broadcast FM signals, it has been suggested that the FM IF frequency be substantially reduced so that the IF filter can be formed as an active filter capable of fabrication on the receiver IC chip. In this regard, it has been proposed to reduce the FM IF frequency from the standard frequency of 10.7 MHz to a low frequency of approximately 70 kHz. But, when the IF frequency is so reduced, the rejection characteristics for rejecting or suppressing a so-called image signal component may be substantially deteriorated.
As used in the specification and claims herein, an "image signal component" refers to an undesired one of a double sideband component produced when an FM (or AM) signal is transmitted. For example, in both FM and AM receiving techniques, a received RF signal of frequency (f0+.DELTA.f) is mixed with a local oscillating signal of frequency f.sub.0 to produce an IF signal of frequency .DELTA.f. The desired RF frequency (f.sub.0 +.DELTA.f) may be accompanied by an image signal component of RF frequency (f.sub.0 -.DELTA.f). When the RF image signal component is subjected to mixing with the local oscillating signal, an IF image signal component of frequency .DELTA.f is produced and this interferes with the desired, or main IF component. As the IF frequency is reduced, such as from 10.7 MHz to 70 kHz for an FM signal, the ability to reject, block or suppress the IF image signal component is reduced.
If the RF image signal component is due to an adjacent broadcast frequency, the narrowing of the frequency separation between adjacent broadcast frequencies may help reduce the possibility of image interference. For example, frequency separation of about 100 kHz, such as found in Japan, results in minimal image interference.
In addition to deteriorating image component rejection, a reduction in the FM IF frequency to 70 kHz may result in an IF bandwidth that is so limited as to be unable to accommodate the full frequency range of FM broadcast signals. For example, stereo separation may be deteriorated, signal distortion may be increased and beat interference may be pronounced, particularly beat interference caused by the typical SCA component (as is known, the SCA component allows broadcasters to provide a subscription background music service). By reducing the FM IF frequency to 70 kHz, the SCA subcarrier, which typically is on the order of 67 kHz, may produce beat interference.
If a comparable reduction in the AM IF frequency is made, the resultant AM IF frequency may reside in the audible band, thereby resulting in beat interference and other difficulties.
Thus, it had been thought heretofore that FM receivers as well as AM receivers, and particularly combined FM/AM receivers, cannot be readily fabricated on a single IC chip because IF filters tuned to the conventional 10.7 MHz and 455 kHz IF frequencies cannot be integrated therein. As a result, ceramic IF filters have been attached externally to the receiver IC chip, resulting in the drawbacks noted above. Alternatively, a significant reduction in the FM and AM IF frequencies which would allow fabrication of the IF filters onto the receiver IC chip results in serious deteriorations of the performance characteristics of the receiver. As mentioned above, image component rejection and stereo separation are reduced while distortion and beat interference are increased.